The human gene for the D-Site Binding Protein (DBP) has been sequenced and characterized. This gene is a member of the b/ZIP family of transcription factors and is one of three genes forming the PAR subfamily. DBP has been implicated in the diurnal regulation of a variety of liver-specific genes. Examination of the genomic structure of DBP reveals that the gene is divided into four exons and is contained within a relatively compact region of approximately 6 kb. These exons appear to correspond to functional divisions of the DBP protein. Exon 1 contains a long 5' UTR, and conservation between the rat and the human genes of the presence of small open reading frames within this region suggests that it may play a role in translational control. Exon 2 contains a limited region of similarity to the other PAR domain genes, which may be part of a potential activation domain. Exon 3 contains the PAR domain and differs by only 1 of 71 amino acids between rat and human. Exon 4, containing both the basic and the leucine zipper domains, is likewise highly conserved. The overall degree of homology between the rat and the human cDNA sequences is 82% for the nucleic acid sequence and 92% for the protein sequence. Comparison of the rat and human proximal promoters reveals extensive sequence conservation, with two previously characterized DNA binding sites being conserved at the functional and sequence levels.